In signal processing, it is essential to convert a continuous-time analog signal to a discrete-time digital signal by sampling and quantization, so the resultant digital signal may be processed by digital signal processing.
However, sampling an analog signal at a sampling frequency will cause undesired spectrum repetitions to appear at integer multiplications of the sampling frequency, and the spectrum repetitions will cause alias. To address the issues, an AAF is utilized before sampling.
To suppress the issues due to sampling, a signal transfer function of an AAF is expected to be low-pass with well-controlled roll-off, and to have notches at integer multiplications of the sampling frequency. However, it is difficult for traditional continuous-time filter to have such notches without suffering from side effects.